Nuclear power will only play a limited role in the world's energy future because of its "absurdly high" cost, Al Gore said on Thursday.

Despite several countries, including the US, UK and China, pushing forward with plans for new nuclear reactors, the former vice-president said the economics of nuclear meant that it was unlikely to play a major role.

Gore's comments seem to suggest he has changed his mind on how attractive nuclear is – in 2009, he said he saw it playing "a somewhat larger role" in the energy mix because of climate change and efforts to cut carbon emissions. "I'm not a reflexive opponent of nuclear. I used to be enthusiastic about it, but I'm now sceptical about it," he told the Guardian at the time.

In the Reddit conversation, he said that while nuclear was expensive, renewable energy technologies were fast becoming cheaper. "Meanwhile, solar PV [photovoltaics] is riding a 'Moore's Law Jr' costdown curve. Wind and efficiency too, though not as steep. We need to get to scale on renewables quickly and make the transition."

Gore also robustly defended climate science when asked "what is the one undeniable scientific fact that you feel backs it [global warming] up the most effectively?". After listing the national science academies and scientists who support the evidence of manmade climate change, he added: "There is as strong a consensus as you will find in science, with the possible exception of the existence of gravity."

He also reiterated his call, in a Guardian interview this week, for Barack Obama to "start with climate" as a priority during his second term. Gore on Tuesday said: ""He has the mandate. He has the opportunity, and he has the inherent ability to provide the leadership needed. I really hope that he will, and I will respectfully ask him to do exactly that."

Gore also called on the Obama administration to push for a carbon tax during negotiations over the so-called fiscal cliff budget crisis.

On Wednesday, Obama said he would take personal charge of finding a solution to action on climate change during his second term.